Jason-CS is the second component of
the hybrid solution (Jason-3 + Jason-CS) agreed to in 2009. Jason-CS
will ensure continuity with Jason-3 to guarantee adequate overlap with
Jason-3. At least two satellites with a 7 years lifetime each (5 years
+ 2 years consumables) are planned to give time before new technologies
such as swath interferometry (SWOT mission) can be considered as
operational. 1)2)

The Jason-CS satellite will carry a
radar altimeter package to continue the high-precision, low-inclination
altimetry missions of Jason-2 and -3. It will complement the
high-inclination measurements on Sentinel-3 to obtain high-precision
global sea-surface topography for the marine and climate user
community.

In late 2013, following a
request from the EC (European Commission), it was agreed that the
Jason-CS mission should become more closely associated with the other
missions in the Copernicus family, and use the name Sentinel-6.
However, there were reasons why the Jason-CS name should be retained. A
compromise was adopted so that the Sentinel-6 mission will be
implemented with the Jason-CS satellite, and partner organizations are
able to use either name according to circumstances.

• As part of the approval
process on the EUMETSAT side, the second meeting of potential program
participants was held in December 2013. At this meeting, ESA announced
that the new High Resolution Microwave Radiometer, which was still
under technical investigation, would be suppressed for affordability
reasons. The detailed technical definition continues in Phase-B2,
including the selection of the subcontractor for the Mono-Propellant
Propulsion System being performed according to ESA’s Best
Practice rules. 3)

• In early December 2014,
ESA selected Airbus Defence and Space as the prime contractor to
develop and construct the first Jason-CS/Sentinel-6 satellite. 4)5)

• On May 11, 2015, ESA
and Airbus Defence and Space signed a contract to develop the Jason-CS
/ Sentinel-6A satellite mission for Europe’s Copernicus program. 6)

• In July 2015, TAS
(Thales Alenia Space) signed the first part of a contract with Airbus
Defense and Space to supply Poseidon-4 spaceborne radar altimeters.
These instruments will be installed on the Jason-CS/Sentinel 6-A and
Jason-CS/Sentinel 6-B satellites developed by Airbus Defense and Space
for ESA (European Space Agency), in collaboration with EUMETSAT and the
European Commission, for the Copernicus program. 7)

- Drawing on a 20 year
heritage of orbital operations, the Poseidon-4 altimeter features
higher performance than the previous generation, because of the
introduction of a new, "interleaved" SAR (Synthetic Aperture Radar)
operating mode. Poseidon-4 will also feature a new architecture,
improving the role of the digital functions to support higher stability
of the performances, and eventually reduce development costs.

• Sept. 11, 2015: The
EUMETSAT Member States have approved the development and implementation
of the collaborative high precision ocean altimetry Jason-CS/Sentinel-6
mission, involving also ESA, the European Union through its Copernicus
program, and the United States, through NASA and NOAA. 8)

The Jason-CS program constitutes
EUMETSAT’s contribution to the Copernicus Sentinel-6 mission to
be developed and implemented through a partnership between the EU, ESA,
EUMETSAT, NASA, and NOAA. From 2020 to beyond 2030, the Sentinel-6
mission will uniquely extend the climate record of sea-level
measurements accumulated since 1992 by TOPEX/Poseidon, Jason-1 ,
Jason-2 , and Jason-3. A prime mission objective is to continue this
long global sea-level time series with an error on the sea level trend
of less than 1mm/year. The Sentinel-6 mission will also be an essential
observing system for operational oceanography and seasonal forecasts in Europe and beyond.
It will provide measurements of sea surface height, significant wave
height, and wind speed without degradation in precision and accuracy
compared to the currently flying Jason-2 mission. As such, like its
predecessors, the proposed mission will provide key user measurement
services for sea-level-rise monitoring, operational oceanography, and
marine meteorology. These services will be aligned with those of the
Sentinel-3 missions, which will be operational in the same era, see
Figure 1. 9)

In addition to the altimeter data
service, Sentinel-6 will also include a GNSS-RO (GNSS Radio
Occultation) instrument as a secondary payload, taking advantage of the
non-sun-synchronous orbit of Sentinel-6. The GNSS-RO measurements will
provide information on atmospheric pressure, temperature and water
vapor as well as ionospheric data. The radio occultation data service
primarily addresses the needs of meteorological and climate users.

The Sentinel-6 mission program
consists of two identical satellites (Jason-CS A and Jason-CS B) with
each a nominal lifetime of 5.5 years and a planned overlap of at least
6 months. The satellites will be launched sequentially into the
“Jason orbit” to take over the services of Jason-3 when
this scheduled mission becomes of age. Currently, the launches of
Jason-CS A and B are planned for 2020 and 2026, respectively.

Figure 3
outlines the multi-partner program and agreement setup underlying the
Sentinel-6 missions. The European contribution will be implemented
through the combination of the EU/ESA Copernicus program and the
optional EUMETSAT Jason-CS program , for the joint benefits of the
meteorological and Copernicus user communities in Europe. In addition,
on behalf of the United States, NASA and NOAA are developing a
dedicated Jason-CS program. The following high-level sharing of
responsibilities is envisaged (which may still be subject to some
changes):

• EUMETSAT is the system
authority and is responsible for the Sentinel-6 ground segment
development and operations preparation. EUMETSAT will also carry out
the operations build-up and operations of the Sentinel-6 system
including both satellites and delivery of data services to Copernicus
service providers and users on behalf of the EU. Additionally EUMETSAT
will fund S-6 B (together with the EU) and potentially part of S6 A as
well.

• ESA is responsible for the
development of the first Jason-CS satellite and the instruments
prototype processors as well as for the procurement of the recurrent
satellite on behalf of EUMETSAT, CNES and the EU. The industrial
consortium strongly based on the CryoSat team. It will operate the
satellite in the first few days after launch, until the basic check-out
of the payload is complete. It is responsible also for the instruments
prototype processors as well as for the procurement of the recurrent
satellite on behalf of EUMETSAT and the EU.

• CNES (France) is providing
expert support to the mission and system development. During operations
will process data from the DORIS
(Doppler-Orbitography-and-Radiopositioning-Integrated-by-Satellite)
payload and provide precise orbits.

• The EU, through the EC
(European Commission), will fund the procurement of S-6 B (together
with EUMETSAT) and the operations for both A and B satellites.

• NASA will deliver the US
payload instruments for both satellites and will provide ground segment
development support, launch services, and contributions to operations.

• NOAA (National Oceanic and
Atmospheric Administration) is providing ground stations to complement
the EUMETSAT station and will process and distribute science data.

• NASA/JPL is developing the US payload instruments and procuring the launcher. NASA will also support the science team.

• The European Space Agency has
selected Airbus Defence and Space as the prime contractor to develop
and construct the two new satellites in Friedrichshafen, Germany.

The three space agencies will share
the responsibility for the science team coordination and the
calibration and/or validation activities, with EC being involved in the
interactions with the science teams. In addition, agreements will be
concluded between EUMETSAT and CNES and between NOAA and NASA for
system and science expertise support.

Mission objectives:

Sentinel-6 will be a truly
operational mission in all aspects of its main user services. Hence,
full emphasis is put on reduction of downtime to a minimum, on timely
distribution of data products, and on high quality and reliability of
the measurement data. The mission will also include support to
information service providers and major reprocessing activities.

The Sentinel-6 product suite is
currently being detailed. The baseline is to provide a product suite
that will enable an optimal combination with products from other
altimeter missions. This is particularly pursued for combining
Sentinel-6 with the Sentinel-3 Ku/C radar altimeter (SRAL) missions.
Next to the conventional Level 2 products, known as GDRs (Geophysical
Data Records) for the Jason missions, the Sentinel-6 product suite will
include Level 1 products aimed at the further study of the intrinsic
altimeter waveforms and development and innovative processing
techniques. Also, the generation of higher-level single-mission
products (Level 2P and Level 3) are supported in order to serve mainly
the ocean modelling community.

Sentinel-6 products are to meet high
standards, such that they will be of sufficient quality to serve as the
high precision reference mission for other altimeter missions. It has
been formally required that the mission performance shall not be worse
than the known performance of Jason-2. With the current design,
however, the expectation is that the Sentinel-6 mission will outperform
Jason-2 on many aspects and will form a reliable state of the art
reference for various other altimeter missions in the near future.

The Sentinel-6 products will also
maintain their quality closer to the coastline than products from its
predecessor Jason missions (e.g. Raney, 1998; Gommenginger et al.,
2012; Halimi et al., 2014). 11)12)13)
This, among other techniques, will be facilitated by replacing the
conventional LRM (Low-Resolution Mode) altimeter by one that has
along-track SAR (Synthetic Aperture Radar) capabilities.

The Sentinel-6 radio occultation
products will contribute to operational weather forecasting and to
assessments of atmospheric climate trends by providing information that
allows to derive atmospheric temperature and water vapor profiles. In
addition, ionospheric data are also provided up to 500 km altitude.

Mission characteristics:

The Sentinel-6 Space Segment
consists of two successive Jason-CS satellites (A and B), based on the
CryoSat-2 heritage platform, with some tailoring to specific needs of
the Sentinel-6 mission. The satellites will embark the following main
payload:

• A radar altimeter
(Poseidon-4), to measure the range between the satellite and the mean
ocean surface, determine significant wave height and wind speed, and
provide the correction for the altimeter range path delay in the
ionosphere by using signals at two distinct frequencies (Ku-band and
C-band).

• A microwave radiometer,
called AMR-C (Advanced Microwave Radiometer-C) of JPL, to provide a
correction for the wet tropospheric path delay for the altimeter range
measurement.

• POD (Precise Orbit
Determination) instruments – namely a GNSS (Global Navigation
Satellite System) and precise orbit determination receiver (GNSS-POD),
a DORIS (Doppler Orbitography and Radiopositioning Integrated by
Satellite) instrument, and a LRA (Laser Retroreflector Array) –
to provide with high accuracy and precision a measurement of the
orbital position as needed for the conversion of the measurement of
altimeter range into a sea level.

The GNSS-RO instrument is added to
Sentinel-6 as a secondary mission to provide radio occultation
observation services to meteorological users. However, the primary
altimeter mission supported by the other instruments takes priority in
all design and mission planning.

It is important to remark that the
Poseidon-4 radar altimeter has evolved significantly from the
Poseidon-3A and -3B instruments on board Jason-2 and -3, respectively.
In addition to a conventional pulse-width limited processing, also
known as low-resolution mode, the Poseidon-4 on board the Jason-CS
satellites will also have the facility of simultaneous high-resolution
(HR) processing, generally referred to as SAR (Synthetic Aperture
Radar) mode processing. This HR processing will provide further service
alignment with the SAR mode of the Sentinel-3 SRAL mission.

The Jason-CS satellites will fly in the same orbit as their predecessors, TOPEX/Poseidon and the Jason missions (Table 3).
This is a non-sun-synchronous orbit with a nominal altitude of 1336 km
and 66º inclination. The orbit period is 112 min and 26 s and the
ground track cycle repeats approximately every 9 days and 22 hours.
Because of the relatively large ground track spacing of 315 km at the
equator, Jason-CS alone will not be able to satisfy the sampling
requirements for mesoscale oceanography. Thus, the Sentinel-6 mission
is coordinated with other altimeter missions, chiefly the Sentinel-3
mission, to provide together a complementary spatiotemporal sampling of
the oceans and serve as a high-precision reference to sea-level-change
studies.

Spacecraft:

ESA has selected Airbus DS as the
prime contractor to develop and construct the two new satellites in
Friedrichshafen, Germany. The development is well advanced and the
project is going into the integration phase. Sentinel-6/Jason-CS
satellites are designed to orbit for minimum 5.5 years each and will
ensure measurements carried out on a continuous basis from 2020
onwards, with better performances in respect to earlier Jason series.
The satellites will measure their distance to the ocean surface with an
accuracy of a few centimeters, from an altitude of 1,336 km (Ref. 10). 14)

Sentinel-6 /Jason-CS will be an
essential observing system for sea-level-rise monitoring, coastal zones
altimetry, operational oceanography, seasonal forecast and marine
meteorology. The two identically equipped A and B satellites are
designed for a mission lifetime of 7.5 years and a planned overlap of
at least 1.5 years. The S-6 satellites will give time before new
technologies, such as the Interferometric Synthetic Aperture Radar
(SWOT mission), will be consolidated (Ref. 9), which is currently expected to happen in the second half of the ‘20 decade.

- Use of off-the-shelf equipments for the platform as far as possible for risk mitigation

- Harsh space radiation environment.

Mechanical Architecture and Configuration: As a result of these conditions a compact satellite body (Figure 4)
has been selected based on the design principles from other missions
designed for drifting orbits, like CryoSat-2. Since the majority of
instruments requires nadir pointing of their antennas and thermal
radiators, the principle dimensions of the satellite structure are
vastly pre-determined by their size.

S-6 has a total length of 5085 mm
(along Xsc), a height of 2349 mm (along Zsc) and a width of 2581 mm
(along Ysc) in stowed configuration. The S/C dry mass with margin, is
1039 kg. The launch mass, including system margin and propellant mass,
is 1362 kg, fully compatible also with the smaller among the proposed
launchers (Antares).

Two fixed Solar Arrays (SA) are
located in the form of a tent. Two additional deployable solar panels
are released by simple passive deployment mechanisms. The distribution
of equipments has been determined mainly by the following constraints:

- Free fields of view for the instruments and short distance between the ones needing stable alignment.

- Short distance for RF path and reduction of RF interferences.

- Accommodation of the high dissipating equipments on a nadir panel and far from alignment critical payload elements.

- Accommodation of the monopropellant fuel tank close to the satellite’s launcher interface.

The POS4 (Poseidon-4 Radar
Altimeter) is the main instrument of the S-6/Jason-CS mission. Its
redundant electronic units are mounted on the nadir pointing Main
Payload Panel, with a large thermal radiator. The antenna itself is
mounted almost isostatically to the Payload main panel that embeds heat
pipes in order to comply with stringent temperature stability
requirements of the Altimeter. The AMR-C Radiometer and the Star
Trackers are mounted on the Payload front panel. The Payload Panel
supporting the redundant RA (Radar Altimeter) is designed as a module
to be assembled and tested independently.

Stability
of alignment between Altimeter antenna, Star Trackers and Radiometer
are guaranteed by the close distance resulting in similar temperatures
and low relative thermal distortions.

The core elements of the satellite
are installed in the bus section, the majority of the instruments
instead are located in the payload section (Figure 5).
These show significant thermal dissipation and unit masses, hence are
accommodated on the dissipating nadir panels to achieve their operating
temperatures and to balance the satellite center of mass. Data exchange
is done with an X-band and an S-band systems located on the nadir
panel. Nearby are located the DORIS receiver and antenna for precise
position determination.

The MPPS (Mono-Propellant Propulsion
System) items are mounted on a separate support structure. Therefore
the MPPS can be assembled and tested separately from the satellite AIT
sequence, then finally inserted into the launcher interface ring
adapter. To cope with the stringent center of mass knowledge
requirement, dedicated metal ring elements are installed inside the
tank to control the gas bubble of the pressurant during the mission.

The redundant European GNSS-POD and
its antennas are accommodated on the zenith panel. Regarding the US
GNSS-RO, one antenna is mounted in zenith direction (GNSS-RO-PA), one
in flight (GNSS-RO fore antenna) and one in anti-flight direction
(GNSS-RO aft antenna).

The
S-6 LRA is accommodated on the nadir plate of the satellite close to
the center of gravity. The REM (Radiation Environment Measurement Unit)
has been lately introduced as experimental payload and placed, outside,
on the front panel. All structure panels are made of aluminum sandwich.
The solar array panels are made of CFRP (Carbon Fiber Reinforced
Polymer) facesheets and aluminum honeycomb.

TCS (Thermal Control Subsystem): The
TCS design of the S-6 satellites incorporates passive and active
elements. The passive elements are MLI (Multi Layer Insulation)
blankets and dedicated radiators covered with SSM (Secondary Surface
Mirrors) providing a rather homogeneous environment for heat rejection
towards Earth. The main structure is partly painted black internally in
order to minimize temperature gradients inside the structure. For
active temperature control, heaters are implemented in dedicated areas.

Electrical and Functional
Architecture: The "Electrical System" of the S-6 satellite comprises
all the necessary hardware to operate the satellite, and to execute the
software. This covers the following functional chains:

The electrical architecture chosen
for S-6 applies the Electrical Interface Standardization for satellite
architectures successfully implemented by Airbus in many recent
programs, and in very close commonality with Sentinel-2 and the Airbus
internal Astrobus concept. The architecture shows compliance at optimal
cost and risk plus demonstrating reliable heritage.

EPS (Electrical Power Subsystem): The EPS generates electrical power in sunlight by operating the 17.5m2
body mounted solar array at its maximum power point. It can provide
nearly 5.5 kW at BOL (Begin Of Life), about 1 kW average in flight. The
EPS manages the charge and discharge of the Li-Ion battery based on
1152 cells, split into two modules, for a total of 147 Ah EOL (End Of
Life).

The unregulated main-bus (29.5 -
33.6 V) is managed according the MPPT (Maximum Power Point Tracking)
concept and the batteries are directly connected to it. Via LCLs
(Latching Current Limiters), the EPS provides main-bus overvoltage and
undervoltage protection and distributes protected unregulated primary
power to all the satellite users. - The EPS provides also a hot
redundant failure handling function, control of the heaters and
passivation at EOL via leak path.

DHS
(Data Handling Subsystem): The DHS is in charge of the overall
satellite command and control including AOCS algorithms. It is running
the on-board SW and FDIR (Fault detection, Isolation and Recovery). The
DHS distributes ground and software issued commands to the satellite
and collects the satellite housekeeping telemetry.

The platform and payload units are
connected with the OBC each through dedicated MIL-buses and to the RIU
(Remote Interface Unit) via discrete I/O interfaces. Direct
telecommands and essential telemetry links are implemented to enable
ground to directly command the various on-board subsystems and units.

The DHS comprises two internally
redundant units, the OBC and the RIU. It includes a small mass memory,
but the main one is a dedicated MMFU that is part of the PDHT system.

The OBC can send HPC-SHP (High
Priority High Power Commands) to various equipments in order to allow
their switching by direct commanding from ground without the need of
software.

The RIU comprises several modules.
While the "Core" part of the RIU is providing the standard I/O I/F,
there are additional modules to control the non-standard functions.

AOCS (Attitude and Orbit
Control Subsystem): The AOCS is responsible for the satellite’s
attitude and orbit control through the following functionalities: rate
damping, vector sun acquisition, safe mode control, fine pointing of
the payloads in nominal mode (with GNSS-POD support) and orbit control
maneuvers.

Several individual sensors and
actuators are necessary to carry out this task: RW, MTQ, CESS, MAG,
RMU, STR and GNSS-POD. Some communicating via the MIL-bus, others via
discrete TM/TC lines.

MPPS (Mono-Propellant
Propulsion Subsystem): The MPPS uses hydrazine propellant. It is
assembled with two independent, cold redundant branches each ending in
four 8 N thrusters. For safety reasons, every thruster has two
independent actuators in series. Each thruster is equipped with two CBH
(Catalyzer Bed Heaters) and a PT 100 thermistor.

PDHT (Payload Data Handling
and Transmission): The PDHT system consists of the internally redundant
MMFU(Mass Memory and Formatting Unit) and XBS (X-band System). The MMFU
is a standalone solid mass memory based on SDRAM (Synchronous Dynamic
Random Access Memory) technology with 352 Gbit EOL capacity. It
receives data from both the RA and the OBC (collecting from all the
other data providers) via SpaceWire links. It manages and stores the
incoming data in packet stores, on APID (Application Process ID) bases,
and allows read and write accesses at the same time. The read data are
formatted and routed on demand to either the XBS sides.

The XBS consists of the redundant
X-band XDA (Downlink Assembly) and the X-band antenna. The XDA
modulates the data onto the X-band carrier for transmission to the
ground, transmitting them at 150 Mbit/s. The XBS is used only for
scientific and telemetry data.

TT&C (Tracking, Telemetry
& Command): The TT&C is a conventional S-band system for
telecommand, telemetry and ranging consisting of two S-band RX/TX
transponders (with a ranging channel), one hemispherical antenna
(nadir) for nominal communications, one hemispherical antenna (zenith)
and one hybrid coupler to simultaneously connect the antennas to both
transponders. It is also used for telemetry data, during LEOP (Launch
and Early Orbit Phase). -The data rates are 16 kbit/s in uplink and 32
kbit/s LR (Low data Rate) or 1 Mbit/s (high data rate, HR) in downlink.

Redundancy concept and implementation:
The essential I/Fs (Interfaces) are double cross-strapped provided
(with nominal and redundant driver and receiver functions, with 2 I/Fs
each and external cross-strap). E.g. MIL and SpW buses. The standard
I/Fs are cross-strapped inside RIU and OBC (with nominal and redundant
driver and receiver functions, with one interface each and internal
cross-strap on master side only). E.g. Discrete High Priority TM/TC. -
A few special actuators are redundant but not cross-strapped.

Satellite SW Systems:
The S-6 software system is distributed across the spacecraft. It
consists of at least 7 different SW systems embedded in different
units:

• OBC SW: it is embedded into
the OBC. It is the master system data management and control unit. The
SW performs the communication with the ground and comprises AOCS,
thermal, system and data handling controls.

• MMFU Control SW: commands, controls and monitors the data flow and storage.

• Star Tracker SW: determines the 3-axes attitude.

• RA instrument Control SW:
schedules the operational modes, executes the acquisition and tracking
algorithms and manages the calibration mode.

• May 4, 2020: During these
unprecedented times of the COVID-19 (Corona Virus Disease-19) lockdown,
trying to work poses huge challenges for us all. For those that can,
remote working is now pretty much the norm, but this is obviously not
possible for everybody. One might assume that like many industries, the
construction and testing of satellites has been put on hold, but
engineers and scientists are finding ways of continuing to prepare
Europe’s upcoming satellite missions such as the next Copernicus
Sentinels. 15)

Figure 8: With liftoff still
scheduled for the end of 2020, the Copernicus Sentinel-6 Michael
Freilich satellite is currently being tested to ensure that it will
withstand the rigors of launch and the harsh environment of space
during its life in orbit around Earth. The constraints imposed by the
COVID-19 crisis mean that there are far fewer engineers in the
cleanroom testing the satellite at IABG’s center in Ottobrunn
near Munich in Germany – but work continues (image credit: Airbus
DS)

-
For example, with liftoff still scheduled for the end of this year, the
Copernicus Sentinel-6 Michael Freilich satellite is currently being
tested to ensure that it will withstand the rigors of launch and the
harsh environment of space during its life in orbit around Earth.

- This new satellite will assume
the role as a reference mission to provide critical data for the
long-term record of sea-surface height measurements.

- As one of the most severe
consequences of climate change, global sea level is rising –
putting millions of people at risk. It is essential to continue
measuring the changing height of the sea surface to monitor this
worrying trend so that decision-makers are equipped to take appropriate
mitigating action.

- The constraints imposed by the
COVID-19 crisis mean that there are far fewer engineers in the
cleanroom testing the satellite at IABG’s center near Munich in
Germany.

- Pierrik Vuilleumier, ESA’s
Copernicus Sentinel-6 mission project manager, said, “The current
situation has meant that many of us are having to follow the test
campaign remotely. Since this is an international mission, people are
scattered across Europe and the US.

- “Remarkably, we have
reached an important milestone completing the acoustic vibration tests,
which simulate the noisy environment of liftoff and ascent through the
atmosphere. This just shows how the team is determined to meet the
launch date in November, despite the difficult circumstances.”

- Copernicus Sentinel-6 is now set
for the next set of tests, which includes the ‘electromagnetic
compatibility’ tests. With these complete, at the end of
September, it will be transported to the Vandenberg Air Force Base in
California for liftoff on a NASA-provided Space-X Falcon 9 rocket.

- Sentinel-6 Michael Freilich is
being jointly developed by ESA, NASA, EUMETSAT (European Organisation
for the Exploitation of Meteorological Satellites) and NOAA (National
Oceanic and Atmospheric Administration), with support from CNES (Centre
National d'Etudes Spatiales).

• On January 28, 2020, NASA
and its partners announced they have renamed a key ocean observation
satellite launching this fall in honor of Earth scientist Michael
Freilich, who retired last year as head of NASA's Earth Science
division, a position he held since 2006.16)17)

Figure 9: A key ocean
observation satellite launching this fall has been named after Earth
scientist Michael Freilich, as announced Jan. 28 by NASA, ESA (European
Space Agency), the European Organisation for the Exploitation of
Meteorological Satellites (EUMETSAT), and the National Oceanic and
Atmospheric Administration (NOAA), video credit: NASA

NASA - along with ESA
(European Space Agency), the European Commission (EC), the European
Organisation for the Exploitation of Meteorological Satellites
(EUMETSAT), and the National Oceanic and Atmospheric Administration
(NOAA) - made the announcement during a special event at the agency's
headquarters.

"This honor demonstrates the
global reach of Mike's legacy," said NASA Administrator Jim
Bridenstine. "We are grateful for ESA and the European partners'
generosity in recognizing Mike's lifelong dedication to understanding
our planet and improving life for everyone on it. Mike's contributions
to NASA - and to Earth science worldwide - have been invaluable, and we
are thrilled that this satellite bearing his name will uncover new
knowledge about the oceans for which he has such an abiding passion."

The Sentinel-6A/Jason CS
satellite, scheduled to launch this fall from Vandenberg Air Force base
in California, will now be known asSentinel-6 Michael Freilich.
The mission aims to continue high-precision ocean altimetry
measurements in the 2020-2030 timeframe using two identical satellites
launching five years apart - Sentinel-6A Michael Freilich and
Sentinel-6B.

NASA and its partners are
developing the mission with support from the Centre National d'Etudes
Spatiales (CNES), France's space agency. Project management is being
provided by NASA's Jet Propulsion Laboratory in Pasadena, California.
ESA is developing the new Sentinel family of missions specifically to
support the operational needs of the European Union's Copernicus
program, the EU's Earth observation program managed by the European
Commission. They will replace older satellites nearing the end of their
operational lifespan to ensure there are no gaps in ongoing land,
atmosphere and ocean monitoring, as well as introduce new monitoring
capabilities.

"Together with other missions
of the European Union's Earth Observation Programme Copernicus,
Sentinel-6 Michael Freilich will contribute to improved knowledge and
understanding of the role of the ocean in climate change and for
mitigation and adaptation policies in coastal areas," said Mercedes
Garcia Perez, head of the Global Issues and Innovation of the European
Union Delegation to the United States. "It will have a large societal
impact worldwide as it supports applications in the area of operational
oceanography, including ship routing, support for off-shore and other
marine industries, fisheries, and responses to environmental hazards.
This new satellite within the Copernicus constellation will be an
additional tool for implementing the European Green Deal to transition
the EU to a carbon-neutral economy."

A secondary objective of the
mission is to collect high-resolution vertical profiles of temperature,
using the Global Navigation Satellite Sounding Radio-Occultation
sounding technique, which derives atmospheric information from analyses
of signals from international Global Positioning System satellites.
Sentinel-6 measurements of temperature changes in the troposphere and
stratosphere will be used by weather agencies worldwide to improve the
accuracy of global forecasts produced by their complex,
state-of-the-art computer models.

The Sentinel-6 Michael
Freilich satellite also will continue the existing 28-year data set of
sea level changes measured from space. Before his retirement, Freilich
was instrumental in advancing the collaborative mission to a critical
stage of development and helping to strengthen its essential
international partnerships.

"This mission demonstrates
what the United States and Europe can achieve as equal partners in such
a large space project. Our suggestion to rename the mission to
'Sentinel-6 Michael Freilich' is an expression of how thankful we are
to Mike. Without him, this mission as it is today would not have been
possible," said Josef Aschbacher, ESA director of Earth Observation
Programmes.

Freilich's career as an
oceanographer spanned nearly four decades and integrated research on
Earth's oceans, leading satellite mission development, and helping to
train and inspire the next generation of scientific leaders. His
training was in ocean physics, but his vision encompasses the full
spectrum of Earth's dynamics.

"Earth Science shows perhaps
more than any other discipline how important partnership is to the
future of this planet," said Thomas Zurbuchen, NASA associate
administrator for Science. "Mike exemplifies the commitment to
excellence, generosity of spirit and unmatched ability to inspire trust
that made so many people across the world want to advance big goals on
behalf of our planet and all its people by working with NASA. The fact
that ESA and the European partners have given him this unprecedented
honor demonstrates that respect and admiration."

During Freilich's NASA tenure,
the agency increased the pace of Earth science mission launches and in
2014 alone sent five missions to space to study our home planet. The
missions balanced many objectives from research to applications and
technology development activities. Freilich also led NASA's response to
the National Academy of Sciences' first-ever Earth Science and
Applications from Space decadal survey in 2007, which expanded NASA's
innovative Earth-observing programs and continues to guide the agency's
global Earth observation efforts.

"My NOAA colleagues and I
enthusiastically support renaming Sentinel-6A after Mike," said Stephen
Volz, assistant administrator for NOAA's Satellite and Information
Service. "This is a fitting honor for a man who helped transform
space-based Earth observation and has brought together the best
contributions from our global Earth science community to improve our
collective understanding of how our planet is changing." NOAA uses data
from missions such as Sentinel-6 in a variety of ways, from monitoring
the rate of global sea-level rise to producing more accurate weather
forecasts.

Freilich also established the
sustained Venture Class program of low-cost space and airborne science
missions that is now a central feature of the NASA Earth Science
Division's portfolio. He pioneered the broad use of the International
Space Station as a platform for Earth-observing instruments, a unique
observing platform for the Earth system. Unlike many of the traditional
Earth observation platforms, the space station orbits the Earth in an
inclined equatorial orbit that is not Sun-synchronous. This means that
the space station passes over locations between 52 degrees north and 52
degrees south latitude at different times of day and night, and under
varying illumination conditions. This is particularly important for
collecting imagery of unexpected natural hazard and disaster events
such as volcanic eruptions, earthquakes, flooding and tsunamis, as well
as for cross-calibrating other satellites in Sun-synchronous polar
orbits.

Freilich also inaugurated a
NASA activity to use data products from private sector, small-satellite
constellations and commercial partners to supplement traditional
government data sources. Under Freilich's leadership, NASA looked at
new ways to carry out its critical mission and established cutting-edge
programs to use small satellites and payloads hosted on commercial
satellites to advance Earth science research and to demonstrate new
technologies.

All told, during Freilich's
time at NASA Headquarters, he oversaw 16 successful major mission and
instrument launches and eight CubeSat/small-satellite launches. The
agency's Earth Science Division has 14 Earth-observing missions in
development for launch by 2023, which includes eight major hosted
instruments on other nations' satellites.

NASA uses the unique vantage
point of space and suborbital platforms to better understand Earth as
an interconnected system for societal benefit. The agency also develops
new technologies and approaches to observe and study Earth with
long-term data records, research, modeling, and computer analysis tools
to quantify how our planet is changing. NASA shares this knowledge with
the global community, including managers and policymakers domestically
and internationally to understand and protect our home planet.

• December 5, 2019: In a
cleanroom in Ottobrunn, Germany, the latest Copernicus Sentinel
satellite is ready for final testing before it is packed up and shipped
to the US for liftoff next year. Designed and built to chart changing
sea level, it is the first of two identical Sentinel-6 satellites that
will be launched consecutively to continue the time series of sea-level
measurements. This new mission builds on heritage from previous ocean
topography satellites, including the French–US Topex-Poseidon and
Jason missions, previous ESA missions such as the ERS satellites,
Envisat and CryoSat, as well as Copernicus Sentinel-3. With millions of
people around the world at risk from rising seas, it is essential to
continue measuring the changing height of the sea surface so that
decision-makers are equipped to take appropriate mitigating action
– as is being currently highlighted at the COP-25 Climate Change
Conference in Spain. 18)

Figure 10: In a cleanroom in
Ottobrunn, Germany, the latest Copernicus Sentinel satellite is ready
for final testing before it is packed up and shipped to the US for
liftoff next year (video credit: ESA)

• November 20, 2019: For the
first time, U.S and European agencies are preparing to launch a 10-year
satellite mission to continue to study the clearest sign of global
warming - rising sea levels. The Sentinel-6/Jason-CS mission (short for
Jason-Continuity of Service), will be the longest-running mission
dedicated to answering the question: How much will Earth's oceans rise
by 2030? 19)

- By 2030, Sentinel-6/Jason-CS will
add to nearly 40 years of sea level records, providing us with the
clearest, most sensitive measure of how humans are changing the planet
and its climate.

- The mission consists of two
identical satellites, Sentinel-6A and Sentinel-6B, launching five years
apart. The Sentinel-6A spacecraft was on display for the media on 15
November for a last look in its clean room in Germany's IABG space test
center. The satellite is being prepared for a scheduled launch in
November 2020 from Vandenberg Air Force Base in California on a SpaceX
Falcon 9 rocket.

-
Sentinel-6/Jason-CS follows in the footsteps of four other joint
U.S.-European satellite missions - TOPEX/Poseidon and Jason-1, Ocean
Surface Topography/Jason-2, and Jason-3 - that have measured sea level
rise over the past three decades. The data gathered by those missions
have shown that Earth's oceans are rising by an average of 3 mm/year.

- Sentinel-6/Jason-CS will continue
that work, studying not just sea level change but also changes in ocean
circulation, climate variability such as El Niño and La
Niña, and weather patterns, including hurricanes and storms.

- "Global sea level rise is, in a
way, the most complete measure of how humans are changing the climate,"
said Josh Willis, the mission's project scientist at NASA's Jet
Propulsion Laboratory in Pasadena, California. "If you think about it,
global sea level rise means that 70% of Earth's surface is getting
taller - 70% of the planet is changing its shape and growing. So it's
the whole planet changing. That's what we're really measuring."

- As the oceans warm, they expand,
increasing the volume of water; the trapped heat also melts ice sheets
and glaciers, contributing further to sea level rise. The rate at which
it is rising has accelerated over the past 25 years and is expected to
continue accelerating in years to come.

- Along with measuring sea level
rise, the mission will provide datasets that can help with weather
predictions, assessing temperature changes in the atmosphere and
collecting high-resolution vertical profiles of temperature and
humidity.

- As with its Jason-series
predecessors, Sentinel-6/Jason-CS will gather global ocean data every
10 days, providing insights into large ocean features like El
Niño events. However, unlike previous Jason-series missions, its
higher-resolution instruments will also be able to provide data on
smaller ocean features - including complex currents - that will benefit
navigation and fishing communities.

Figure 11: The
Jason-CS/Sentinel-6 mission that will track sea level rise, one of the
clearest signs of global warming, for the next 10 years. Sentinel-6A,
the first of the mission's two satellites, is shown in its clean room
in Germany and is scheduled to launch in November 2020 (image credit:
IABG)

•
November 15, 2019: Media representatives and mission partners gathered
today in Germany to see a new satellite, which will take the lead in
charting sea-level change, before it undergoes final testing and is
packed up for shipment to the US for lift-off next year. 20)

- Copernicus Sentinel-6 was on full
display at the IAGB space test center in Ottobrunn near Munich, giving
media and partners in the mission a unique opportunity to see this
remarkable new satellite up close.

Figure 12: The Copernicus
Sentinel-6/Jason CS stands on display at the IAGB space test center. It
will map up to 95% of Earth’s ice-free ocean every 10 days in
order to monitor sea level variability. The radar altimeter will also
measure the ocean surface topography – the hills and valleys of
the ocean – that help us to map ocean currents. In addition, it
will provide estimates of wind speed and wave height for maritime
safety (image credit: ESA, S. Corvaja)

- ESA’s Director of Earth
Observation Programs, Josef Aschbacher, said, “We are all
extremely proud to see the complete satellite on show here in the
cleanroom. With global sea level rising at shocking rates, Copernicus
Sentinel-6 will take the lead in providing systematic measurements of
sea level so that the worrying trend in sea-level rise can be closely
monitored and key information provided for important policy
decisions.”

- Sentinel-6 is realized thanks to cooperation between ESA, NASA, the European Commission, EUMETSAT and NOAA.

- “The mission has been
developed thanks to the outstanding international cooperation with our
US partners. Sentinel-6 is indeed a model case of pan-European and
US–European cooperation, taking advantage of a 26-year history in
altimetry measurements from space on both sides of the Atlantic.”

- Sentinel-6 builds on heritage
from previous ocean topography satellites, including the
French–US Topex-Poseidon and Jason missions, previous ESA
missions such as the ERS satellites, Envisat and CryoSat, as well as
Copernicus Sentinel-3.

- These missions have shown how sea
level rose by about 3.2 mm on average a year between 1993 and 2018, but
more alarmingly, that the rate of rise has been accelerating over the
last few years. It is now rising at 4.8 mm a year.

- Caused mainly by warming ocean
waters, melting glaciers and diminishing ice sheets, sea-level rise is
one of the most severe consequences of climate change. With millions of
people around the world at risk from rising seas, it is essential to
continue measuring the changing height of the sea surface so that
decision-makers are equipped to take appropriate mitigating action.

- The Copernicus Sentinel-6
satellite will be launched in November 2020 from the Vandenberg Air
Force Base in California, US on a Falcon-9. It will be the first time
ESA cooperates, through NASA, with the private US aerospace
manufacturer SpaceX, which was founded in 2002 by Elon Musk.

• September 3, 2019: Airbus DS
has completed the ocean satellite ‘Copernicus Sentinel-6A’,
and is now sending it on its first journey. Its destination: Ottobrunn
near Munich in Germany, where over the next six months the satellite
will undergo an extensive series of tests at Industrieanlagen
Betriebsgesellschaft mbH (IABG) to prove its readiness for space. 21)22)

- ‘Copernicus
Sentinel-6’ will carry out high-precision measurements of ocean
surface topography. The satellite will measure its distance to the
ocean surface with an accuracy of a few centimeters and, over a mission
lasting up to seven years, use this data to map it, repeating the cycle
every 10 days. It will document changes in sea-surface height, record
and analyze variations in sea levels and observe ocean currents. Exact
observations of changes in sea-surface height provide insights into
global sea levels, the speed and direction of ocean currents, and ocean
heat storage. These measurements are vital for modelling the oceans and
predicting rises in sea levels.

- The findings will enable
governments and institutions to establish effective protection for
coastal regions. The data will be invaluable not only for disaster
relief organizations, but also for authorities involved in urban
planning, securing buildings or commissioning dykes.

- Global sea levels are currently
rising by an average of 3.3 mm/year as a result of global warming; this
could potentially have dramatic consequences for countries with densely
populated coastal areas.

-
Two Sentinel-6 satellites for the European Copernicus Program for
environment and security are currently being developed under
Airbus’s industrial leadership. While it is one of the European
Union’s family of Copernicus satellite missions, Sentinel-6 is
also being realized thanks to an international cooperation between ESA,
NASA, NOAA and EUMETSAT.

- Each satellite has a mass of
approximately 1.5 tons. From November 2020, Sentinel-6A will be the
first of the two Sentinel-6 satellites to continue collecting
satellite-based measurements of the oceans’ surfaces, a task that
began in 1992. Sentinel-6B is then expected to follow in 2025.

• April 12, 2019: Records show
that, on average, global sea level rose by 3.2 mm a year between 1993
and 2018, but hidden within this average is the fact that the rate of
rise has been accelerating over the last few years. Taking measurements
of the height of the sea surface is essential to monitoring this
worrying trend – and the Copernicus Sentinel-6 mission is on the
way to being ready to do just this. 23)

- The mission will be a constellation of two identical satellites that are launched sequentially.

- Over the next decade, the
Copernicus Sentinel-6A and then Sentinel-6B satellites will,
importantly, take the role as reference missions, picking up the task
of continuing the long-term record of sea-surface height measurements
that have so far been supplied by the French–US Topex-Poseidon
and Jason missions.

Figure 14: Copernicus Sentinel-6
radiometer integration. The AMR-C (Advanced Microwave Radiometer for
Climate monitoring) is being integrated on to the Copernicus
Sentinel-6A satellite. The photo shows teams at Airbus in
Friedrichshafen, Germany, lowering the instrument on to the satellite
prior to mechanical mounting and alignment checks. As part of the
international cooperation for this mission, the radiometer has been
supplied by NASA/JPL. The satellite’s main instrument is a radar
altimeter to measure sea-surface height. The radiometer accounts for
the amount of water vapor in atmosphere, which affects the speed of the
altimeter’s radar pulses (image credit: Airbus)

-
The Copernicus Sentinel-6 satellites will each carry a radar altimeter,
which works by measuring the time it takes for radar pulses to travel
to Earth’s surface and back again to the satellite. Combined with
precise satellite location data, altimetry measurements yield the
height of the sea surface.

- Over the next decade, the
Copernicus Sentinel-6A and then Sentinel-6B satellites will,
importantly, take the role as a reference mission, picking up the task
of continuing the long-term record of sea-surface height measurements
that have so far been supplied by the French–US Topex-Poseidon
and Jason missions.

- The Copernicus Sentinel-6
satellites will each carry a radar altimeter, which works by measuring
the time it takes for radar pulses to travel to Earth’s surface
and back again to the satellite. Combined with precise satellite
location data, altimetry measurements yield the height of the sea
surface (Figure 17).

- With Copernicus Sentinel-6A
scheduled for liftoff at the end of next year, the satellite is
currently being equipped with its measuring instruments, which also
include an advanced microwave radiometer at Airbus’ facilities in
Friedrichshafen in Germany.

- The radiometer accounts for the
amount of water vapor in atmosphere, which affects the speed of the
altimeter’s radar pulses. While it is one of the European
Union’s family of Copernicus satellite missions, which all
deliver a wealth of information for a number of environmental services,
Copernicus Sentinel-6 is also being realized thanks to cooperation
between ESA, NASA, NOAA and EUMETSAT.

- As part of this international cooperation, the Copernicus Sentinel-6 radiometer has been supplied by NASA.

- ESA’s Copernicus Sentinel-6
mission scientist, Craig Donlon, said, “The advanced microwave
radiometer has been designed to make sure that the measurements from
Copernicus Sentinel-6 will be of the highest quality to monitor changes
in global sea level and ensure a complete record of sea level for the
coming decades.”

- Pierrik Vuilleumeir, ESA’s
Copernicus Sentinel-6 project manager, added, “We are very happy
with progress so far and, in fact, both satellites are being built in
parallel. We are now looking forward to the next step, which will be to
complete the satellite with the altimeter and the precise orbit
determination instruments. The satellite will then be put through
testing, which includes simulating the vibrations and temperature
during liftoff and also the environment of space for its life in orbit
around Earth.”

•
August 30, 2018: The integration of Sentinel-6A, the first of two
satellites to continue measuring sea levels from 2020, has reached a
new milestone and its critical phase: the propulsion module has been
“mated” with the main structure of the satellite at Airbus.
24)

- In a complex operation, the
Airbus satellite specialists hoisted the approximately 5 m high
satellite platform with pin-point precision over the drive module,
which had already been positioned (Figure 16).
The two components were then fixed in place and assembled. Before this
could happen, the propulsion module, which includes the engines,
control devices and a 240 liter tank with an innovative fuel management
system, had to undergo technical acceptance, since this subsystem can
no longer be accessed once it has been integrated. The propulsion
module now needs to be ‘hooked up’, which will then be
followed by the system tests.

- Two Sentinel-6 satellites for the
European Copernicus Program for environment and security, headed by the
European Commission and ESA, are currently being developed under
Airbus’ industrial leadership, each weighing roughly 1.5 tons.
From November 2020, Sentinel-6A will be the first to continue
collecting satellite-based measurements of the oceans’ surfaces,
a task that began in 1992. Sentinel-6B is then expected to follow in
2025.

-
Sentinel-6 is a mission to carry out high-precision measurements of
ocean surface topography. The satellite will measure its distance to
the ocean surface with an accuracy of a few centimeters and, over a
mission lasting up to seven years, use this data to map it, repeating
the cycle every 10 days. It will document changes in sea-surface
height, record and analyze variations in sea levels and observe ocean
currents. Exact observations of changes in sea-surface height provide
insights into global sea levels, the speed and direction of ocean
currents, and ocean heat storage. The measurements made are vital for
modelling the oceans and predicting rises in sea levels.

- These findings enable governments
and institutions to establish effective protection for coastal regions.
The data is invaluable not only for disaster relief organizations, but
also for authorities involved in urban planning, securing buildings or
commissioning dykes. - Global sea levels are currently rising by an
average of 3 mm/ year as a result of global warming; this could
potentially have dramatic consequences for countries with densely
populated coastal areas.

• September 2017: The satellite
CDR (Critical Design Review) took place, enabling the project to move
into the production Phase-D. Most flight hardware is being manufactured
and satellite integration will start in September 2017. Joint
activities with the NASA, NOAA and Eumetsat partners are proceeding.
Working groups have been formed to address the system engineering and
mission performance aspects. The independent Mission Advisory Group
advising the project partners on scientific issues specific to the
Sentinel-6/Jason-CS mission had its first meeting in June. 25)

Launch: A launch of
Sentinel-6 Michael Freilich mission is scheduled for November 2020.
NASA will provide the payload and launch the Sentinel-6A and -6B
satellites.

In October 2017, NASA selected
SpaceX of Hawthorne, California, to provide launch services for the
Sentinel-6A mission. The launch is currently targeted for November
2020, on a SpaceX Falcon 9 Full Thrust rocket from SLC-4E (Space Launch
Complex 4E) at Vandenberg Air Force Base in California.26)

Orbit: The nominal orbit for S-6 is
the same of the precedent missions (TOPEX/Poseidon, Jason-1 to -3)
ensuring data consistency with the previously acquired time series. The
Jason missions operate from a relatively high altitude (1336 km)
prograde orbit with an inclination of 66º. The main orbit
parameters are reported in Table 3.

Semi-major axis, eccentricity

7714.432261 km, 0.000094

Argument of perigee, inclination (non-sun-synchronous)

270.8268º, 66.034º

Reference altitude (equatorial)

1336 km

Right ascension of ascending node (Ω)

36.411208

Longitude of ascending node (pass 1)

99.924305º

Argument of perigee (ω)

90.0º

Nodal period, orbits per day, repeat cycle

6745.72 s (112 m 23 s), 12.81, 9.91564 days

Number of orbits per cycle, number of passes per cycle

127, 254

Ground track separation at equator, acute angle at equator crossings

315 km, 39.5º

Orbital velocity, ground track velocity

7.2 km/s, 5.8 km/s

Table 3: Parameters of the Sentinel-6 Michael Freilich orbit

Kiruna and Fairbanks (with Wallops
as backup) are chosen as S- and X-band ground stations for sizing
purposes but do not necessarily represent the final choice. Figures 18 and 19
show the intersections of reception cones of exemplary ground stations
and the S-6 ground track. Considering the exemplary ground stations,
the mean contact time will be 16 min with 76 min contact gap.

The Sentinel-3 SRAL (SAR Radar
Altimeter) derived RA (Radar Altimeter) is the principle payload
instrument; its scope is measuring geophysical parameters (SSH, wind
speed and SWH). To retrieve these data, additional information is
required from a number of supporting instruments: a DORIS receiver
(recurrent from CryoSat-2) to enable precise orbit determination and a
Microwave Radiometer to provide the measurement of water vapor
necessary to correct the altimeter data. Orbit tracking data are also
provided by a GPS receiver (partially recurrent from Sentinel-3b and
that in its own right can is capable of POD), and a LRA (Laser
Retro-Reflector), that supports POD. Star trackers are used to meet the
science objectives needed from the altimeter SAR data. An additional
GPS receiver, GNSSRO, provided by NOAA and developed by NASA/JPL, will
be dedicated to radio-occultation measurements.

The POS4 (Poseidon-4) architecture
is composed of two cold redundant DPUs (Digital Processing Units), two
cold redundant RFUs (Radio Frequency Units), a dual-frequency antenna
and three RF Switches. With the improvements of the SAR method over
pulse-width limited processing demonstrated, the agencies requested
industry to investigate the possibility of operating both SAR and
pulse-width limited modes at the same time over all open ocean
improving science return. The pulse/burst characteristics of this new
mode of operation, named ILM (InterLeaved Mode, open burst scenario),
allows the reference LRM (Low Resolution Mode) data series to be
continued , whilst providing science users with a unique global data
set with reduced uncertainties (SAR).

Pulse-width limited LRM data are
obtained from single pulse/burst allowing retrieval of geophysical
parameters (elevation, wind speed and SWH) over single shot scales
between about 1 km and 5 km (Figure 21
left). It has to be noted that the footprint is not only proportional
to the satellite altitude and pulse length, but also to the SWH, due to
non-nadir reflections (e.g. without waves the Jason-1/2 footprint is 2
km, with 15 m waves it becomes about 12 km.

Filtering the data acquired along
the track wrt the rates of change of phase (SAR), are obtained slices
of range rings coherent in phase between them (Fig. 21 right). This allows improvement of the along-track resolution to about 300 m, independently of the SWH.

When all available beams are
collected, the range is corrected for Doppler Shift effects and range
migration, they can be output as a stack file and multi-looked to form
the Level 1b echo waveform. Artificially focusing the echoes (Figure 22) improves the overall SNR (rejecting all reflections from non-nadir sources) and thus improves performances.

Figure 22: SAR echoes focusing along the track (image credit ESA)

The SNR can be even more improved by
averaging pulses since the noise on the signal is independent in each
gate until a limit defined by the Walsh PRF (Pulse Repetition
Frequency) bound. In order to increase the number of echoes per unit
time the transmit bursts are interleaved with receive bursts in what is
known as Open Burst transmission, Figure 23, third chronogram.

The cons of the Open Burst
transmission vs the Closed Burst are: that the data volume and the
power demand is increased and it is needed to vary the PRF (Pulse
Repetition Frequency) around the orbit.

The RA PRF is fixed during an Rx
tracking cycle but adjusted along the orbit (around 9.1 kHz) to cope
with the altitude changes. Therefore, the PRF is constant in reception
to avoid a Tx/Rx pulse overlap. To assure continuity, the PRF is a
close multiple of the Jason-3 one: LRM is an embedded subset
(decimation). The fact that the S-6 PRF is lower than the S-3 one
(Figure 23) will allow a better
sensibility determining the range delay of the leading edge of the
echoes. This is so because the impulse widening (Figure 24)
will be less severe therefore almost all off-nadir Doppler beams will
be useable (e.g. not too broad to resolve low SWH due to “toe
effect”).

To reduce the large SAR data volume
produced on-board, an on-ground-reversible RMC (Range Migration
Correction) is implemented, whose effects can be seen in Figure 25.
The useful range is more or less reduced to about half. As a
consequence, the data rate is reduced by a factor of 2. The process is
reversed on-ground.

Figure 25:
2D FFT (Fast Fourier Transform) of a raw burst power, before (left) and
after (right) RMC processing. High power is red and low power (in
practice thermal noise) is dark blue (image credit ESA)

The DPU handles several measurements
modes. One is the Acquisition Mode, operating only in Ku-band to look
for echo in a defined altitude range. The other modes are combinations
of Open and Closed Loop mode with generation of LRM or SAR or raw SAR
I&Q data or combination of these, see Table 4.

The Closed Loop tracking mode makes
use of an automatic echo recognition and tracking. Instead the Open
Loop tracking mode exploits an on-board DEM (Digital Elevation Map) to
adapt the PRI (PRI=1/PRF) for the elevation of inland waters.

Nominally, the following modes are used, according to Table 4 indications:

In the measurement modes, each of the red lines in Figure 23
is made by combination of Ku & C-band pulses which pattern varies
depending on the mode. One echo is received every pulse transmission at
fixed PRI (~1/9.1 kHz). The majority of these are measurement pulses
but there are also C-band pulses transmitted in order to retrieve a
correction for ionospheric path delay, CAL pulses to trace the
instrument behavior.

A blanking capability is part of the
baseline design of POS-4 and AMR-C as any RFI (Radio Frequency
Interference) between both would lead to performance impacts.

In
addition, the instrument design relies on state of the art digital
hardware improving on-board calibration strategy whilst reducing the
manufacturing time.

The current POS-4 design theoretical performances have been assessed and are provided in Figure 26 that demonstrates S-6 will improve on its required performances for both LRM and SAR processing.

AMR-C is a 3-frequency radiometer
provided by NASA/JPL (funded by NOAA) enhanced to minimize the effects
of instrument drift which is a key design driver for overall mission
success. 27) It will be a direct successor to AMR instrument on Jason-3. 28)

AMR-C comprises a nadir-viewing
offset Gregorian telescope with a one meter primary aperture feeding a
single broad-band corrugated horn, followed by an Orthomode Transducer
and two identical three-band radiometers observing the H and V linear
polarizations of emission from the ocean surface and the atmosphere.
One radiometer is treated as a cold spare. The radiometer is calibrated
every second via a Dicke switch and a calibration noise source
employing noise diodes.

Once per month, a cold sky
calibration is expected to be executed rotating the satellite with a
pitch maneuver (nose up). It is also possible to execute a
supplementary calibration by rotating the secondary reflector about the
axis of the feed horn to focus on sequentially a reflector pointing to
cold space or a warm calibration target. The calibrations are done when
the satellite is over land. - The electronics and feed horn are
thermally stabilized via a circuit controlled by the spacecraft and a
radiator that dissipates heat excesses.

The signals measured are noise power
expressed as a noise temperature in units of Kelvin. The measured noise
temperature referenced to the AMR / HRMR feeds are referred to as the
TA (Antenna Temperature).

An Antenna Pattern Correction is
applied to the TA measurements to subtract noise temperature
contributions from outside the main beam, yielding the Level 1 data
product, main beam brightness temperature. A retrieval algorithm using
empirically derived coefficients yields the Level 2 data product, the
wet path delay estimate (cm) used in the RA range correction.

The AMR-C brightness temperatures
are not only used for the RA water vapor path delay correction but are
also fundamental climate data record from which are derived ocean
measurements of wind speed, water vapor, cloud liquid water, rain rate,
and sea surface temperature.

The AMR-C receiver is based on
heritage from the previous missions with addition of a HRMR (High
Resolution Microwave Radiometer) and a SCS (Supplemental Calibration
System). The radiometer channels at 18.7 GHz, 23.8 GHz, and 34.0 GHz
are inherited from previous AMRs and constitute the radio frequency
subassembly (RFA). The 18.7 GHz channel estimates ocean surface
components in observed brightness temperature, the 23.8 GHz channel
estimates water vapor, and the 34.0 GHz channel estimates cloud liquid.
HRMR consists of bands at 90 GHz, 130 GHz, and 168 GHz. The SCS is an
additional calibration system in order to meet the level 3 payload
requirement of long term radiometric stability. In addition to the RFA,
HRMR, and SCS subassemblies, the AMR-C instrument also contains a
parabolic mirror in the Reflector Subassembly (RSA), and the
Electronics Unit (EU) in the Electronics Subassembly (ESA). 29)

A block diagram of the AMR-C instrument is shown in Figure 28.
HRMR sits at the focus of the primary reflector and the lower frequency
channels in the RFA are offset. There are two identical lower frequency
radiometer units in the AMR-C system, a nominal unit (H-polarization)
and a redundant unit (V-polarization) shown in green. All three of
these receivers have a separate EU containing the Power Converter Unit
(PCU), a Data Acquisition and Control Unit (DAC), and a Housekeeping
Unit (HKU). The DACs of the AMR-H and AMR-V units are crossed-strapped
to the SCS shown in purple, which has fully redundant Control Mechanism
Interface Electronics (CMIE) units, both of which can control either or
both motors in the Standard Dual Drive Actuator (SDDA). Please note
that crossstrapping in Figure 28 is only shown for the EU-H unit to reduce clutter in the figure. HRMR is in turquoise.

AMR-H and AMR-V Receiver Design:
Signal is relayed to the receiver through a circular feed horn. The
signal is split by the Ortho-mode Transducer (OMT) into H and V units,
nominal and redundant, respectively, although the polarization is
arbitrary. The redundant unit will be used as a cold spare. From the
OMT a diplexer divides the signal into 18/24 GHz and 34 GHz channels
and the 18/24 GHz channel is then spilt into separate 18 and 24 GHz
channels. A detector diode along with an ADC converts the signal to a
digital signal, which is then relayed to the spacecraft and transmitted
to the ground. A model of the receivers is shown in Figure 29 and an internal block diagram is shown in Figure 30.
In operation, a Dicke switch at the receiver waveguide output toggles
between the antenna signal and 50 W load for a differential
measurement.

Figure 29: Top and bottom views of the AMR receivers for 18/24 and 34 GHz (image credit: NASA/JPL)

Parameter

Input Return Loss (over channel passbands)

≥15 dB

Dicke Switch Isolation

≥30 dB

Channel Center Frequency

18.7 GHz

23.8 GHz

34.0 GHz

Center Frequency Tolerance

±50 MHz

±100 MHz

±100 MHz

Center Frequency Knowledge

±20 MHz

±20 MHz

±50 MHz

Channel Noise Bandwidth

200 MHz

400 MHz

700 MHz

Noise Bandwidth Tolerance

±50 MHz

±100 MHz

±150 MHz

Passband Ripple

±1 dB max

±1 dB max

±1 dB max

Stopband Rejection

>50 dB

>50 dB

>50 dB

System Noise Figure

≤ 6.2 dB

≤ 6.5 dB

≤ 6.6 dB

System Gain/Temperature Coefficient

≤0.2 dB/ºC

≤0.2 dB/ºC

≤0.2 dB/ºC

Post-detector Circuit Video (3 dB) Bandwidth

≥ 75 kHz

≥ 75 kHz

≥ 75 kHz

Backend Noise (relative to radiometric noise)

≤ 1/3

≤ 1/3

≤ 1/3

Input Dynamic Range

2.7 to 750 K

Digitizer Sampling Rate

≥ 200 ksample/s

Table 5: Level 6 AMR instrument requirements

Figure 30: AMR receiver block diagram (image credit: NASA/JPL)

A fully characterized noise source
at the input of each receiver is used for internal gain stability
calibration. Each noise source contains 3 sets of redundant diodes that
can be used separately or together. A block diagram for the noise
source is shown in Figure 31. The noise
signals are coupled at the receiver input using a directional coupler.
The level 6 receiver requirements flow from the level 4 instrument
requirements. These requirements are summarized in Table 5.

HRMR Receiver Design:
Previous AMRs were limited to a 25 km diameter footprint on the ocean.
In order to provide higher spatial resolution to improve the coastal
zone measurement accuracy to a 3-5 km diameter footprint, a THz
radiometer, HRMR, has been added to the AMR-C instrument. HRMR includes
receiver bands at 90 GHz, 130 GHz, and 168 GHz and is based on
radiometers designed for airborne and cubesat missions, the HAMMER
(High-frequency Airborne Microwave and Millimeter-wave Radiometer), 30) and the TEMPEST (Temporal Experiment for Storms and Tropical Systems), respectively. 31)
HRMR has been designed to attach to three mounting points at the focus
of the RSA to minimize AMR beam blockage. The feedhorn and millimeter
wave modules will be assembled and delivered on a radiatively-cooled
plate, which will be enclosed for better thermal shielding.

HRMR will interface with EU hardware
identical to the AMR units through its digitizer driver unit (DDU).
This receiver utilizes low noise, high gain Indium Phosphide (InP)
MMICs to amplify incoming signal in order to detect it. 32)
Like the AMRs, HRMR signal is relayed through a feedhorn into diode
detectors for each frequency. The calibration noise source is
integrated in the multi-chip module (MCM). It has two noise diodes and
directional couplers to provide stable calibration references.
Additional calibration and stability is provided by the integrated
Dicke switch that toggles between the antenna and reference load at 2
kHz rate to reduce NEDT, see Figure 38. A model of the HRMR receivers is shown in Figure 32 and design parameters are shown in Table 2.

Parameter

Requirement

Channel Center Frequency

90 GHz

130 GHz

168 GHz

Center Frequency Tolerance

±5 GHz

±5 GHz

±5 GHz

Minimum Bandwidth

5 GHz

5 GHz

5 GHz

Noise Temperature

2000 K

2500 K

3500 K

Brightness Temperature Sensitivity

0,2 K

0.2 K

0.2 K

Deviation from White Noise Level Over 60 secs

<0.2 K

<0.2 K

<0.2 K

Table 6: HRMR receiver design parameters

Figure 32: Top and bottom views of the HRMR receiver (image credit: NASA/JPL)

The SCS (Supplemental Calibration System): Due to long term fluctuations seen in the noise source from the Jason-3 mission 33)
a SCS has been included on AMR-C. This subsystem is designed to turn
the secondary mirror every 5-10 days so that the AMR receivers look at
a warm load at ambient temperature (~200 K) and a cold load (cold sky,
~3 K), shown in Figure 33. As shown in Figure 28,
the SCS only calibrates the AMR receivers, not HRMR, whose signal path
is instead at the focus of the primary. These calibrations will be done
over land in order to maximize observation times over the ocean.

The SCS is driven by an SDDA motor,
which is a block redundant, single fault tolerant mechanical/electronic
assembly that provides a rotary output with fully characterized torque,
speed, and current relationships. The gearbox couples dual spur gears
for the first stage with dual harmonic gears in the final stage. The
redundancy in the SDDA means that no single mechanism failure within
the assembly will prevent the output from rotating. The SDDA power is
supplied separately from the rest of the instrument. The mechanism
control is cross-strapped to both the H and V flight computers. During
launch the secondary mirror is held in place by the Launch Lock
Mechanism (LLM).

Figure 33: The SCS, which rotates a secondary mirror to look at ambient and cold calibration targets (image credit: NASA/JPL)

Initial results

Thermal Modeling: The AMR-C
instrument will have a PID-controlled thermal loop run by the
spacecraft. The preliminary thermal design was simulated using a
P-regulator and modeling shows that the receiver will meet its thermal
requirements detailed below. The thermal analysis was done for three
different cases: a hot winter, a hot summer, and a cold summer. Results
are shown for several simulations lasting the duration of one orbit,
which is 112 minutes long. Figure 34
models the AMRH receiver thermal stability over one orbit showing that
it can be kept to within ~0.04 °C/min. Similarly, Figure 35
shows the modeled thermal stability for HRMR. HRMR has no requirement,
but the goal for this receiver is ≤ 0.1 °C of variation over an
orbit. Peaks and minimums in these models are a result of the
satellite’s orbit as it transitions in and out of the sun. In
Figure 36, models show the thermal variation within the AMR-H receiver will be ± 2.5 °C. Figure 37
shows the thermal variations between the feed horn assembly (FHA) and
the AMR-H receiver. The requirement is these thermal variations not
exceed 10 °C and models show that this difference is well within
the model’s margin.

Figure 34: AMR-H receiver thermal stability can be kept to less than 0.04ºC/min during an orbit (image credit: NASA/JPL)

Figure 37: The thermal variations between the feed horn and the receiver over one orbit (image credit: NASA/JPL)

HRMR Prototype: The HRMR 90
GHz prototype’s measured noise temperature is ~500 K. The noise
equivalent differential measurement (NEDT) was measured for both 90 and
160 GHz. The NEDT is a measure of sensitivity that determines the
threshold for the minimum differential temperature that the system can
detect. This measurement is taken by looking at the difference between
the receiver looking at a blackbody radiator and a 50 W reference load
using a Dicke switch. The results of the NEDT measurements for 90 and
160 GHz prototype receivers are presented in Figure 38.
The NEDT at 90 GHz is in green and the NEDT at 160 GHz is in blue. At
the Dicke switch frequency of 2 kHz, the NEDTs ~0.1 K, which provides a
50% margin on the sensitivity requirement.

Further measurements made on the
prototype indicate that the power and mass are within the margins of
their allotted budgets. These results are presented in Table 7.

The AMR-C team plans to deliver two
flight instruments, one for each mission ~5 years apart. The instrument
has passed the preliminary design review (PDR) and Phase C has begun.
Hardware testing will begin in the summer of 2017 and the critical
design review (CDR) will be in the fall of 2017. Instrument I&T for
the first flight module will start in the Spring of 2018 for delivery
to payload I&T in early 2019. Instrument I&T for the second
flight module will begin in early 2019 after the delivery of the first
flight module, and begin payload I&T in fall 2019. Sentinel-6 is
expected to launch in 2020.

DORIS

The DORIS DGXX-SEV receiver, carried
on-board S-6, is a direct evolution of the DGXX-S of Jason-3. It is
part of an overall system which is able to provide tracking
measurements for precise orbit determination, and time-transfer. The
DORIS system comprises a network of 55 ground beacons, a number of
receivers on several satellites in orbit and in development, and
ground-segment facilities. It is part of the IDS (International DORIS
Service), which also offers the possibility of precise localization of
user-beacons.

DORIS is an up-link radio frequency
tracking system based on the Doppler principle. Each beacon in the
ground network broadcasts stable two frequencies, at S-band and VHF
(2036.25 MHz and 401.25 MHz respectively). Every 10 seconds the
receiver delivers the Doppler shift data calculated using the on-board
ultra-stable oscillator (USO with a stability of 5 x 10-13
over 10 to 100 seconds) as a reference; essentially this enables the
line- of-sight velocity to be determined. The use of two frequencies
allows the ionospheric effects to be compensated and also enables the
ionospheric total electron content to be estimated. The set of radial
velocities from the dense network of precisely located beacons is rich
set of tracking data.

The DORIS’s USO sync signal is used also to drive the GNSS-POD and RA pinpointing on the on-board DEM used in Open Loop.

The DORIS instrument consists of a
receiver and processing unit (BDR), which is composed of 2 identical
functional chains in cold redundancy. Both share a common RF signals
distribution unit (DRF) which also contains a (cold) redundant USO
(Ultra Stable Oscillator). And a dual-frequency antenna.

The DORIS system includes the
possibility of encoding information on the uplinked signals, and three
privileged master beacons, at Toulouse, Kourou and Papeete, provide
such uplink services. Data uplinked from these stations (which is
updated weekly and used by all DORIS instruments in orbit) include the
coordinates of the stations, earth rotation parameters, etc.

The DORIS is not only used for POD,
but also for geodesy and geophysics applications: measuring the
continental drift, fitting the local geodesic network, monitoring the
geophysical deformations, determining the rotation and the gravity
parameters of the Earth and contributing to the international reference
system. 34)

The GNSS-POD receiver is a recurring
PODRIX model in common procurement of the Sentinel program (S-1, S-2,
S-3 and S-6). A PODRIX unit is a multi-constellation (GPS &
Galileo) multi-frequency (L1/E1, L2 and L5/E5a) GNSS receiver. The GNSS
on-board system is composed of two cold-redundant receivers, each
including one tri-frequency (L1/E1, L2 & L5/E5a) receiver with 16
dual frequency channels. Two Extended Patch Excited Cup POD antennas
are provided, one per single electronic box.

GNSS-RO (GNSS-Radio Occultation)

GNSS-RO is a CFI from NASA/JPL. As a
secondary mission instrument, it is used to measure physical properties
of the atmosphere such as temperature, pressure and water vapor, via
detecting the occultation of GNSS signals as they pass through the limb
of the atmosphere.

To measure radio occultations, three
antennas are necessary. One antenna is used for POD, while two other
antennas are directed at the Earth’s limb to collect RO data. One
of these antennas faces the fore of the spacecraft while the other
faces the aft. These antennas enable tracking of the highly defocused
rapidly shifting in frequency GNSS signal passing through the lower
regions of Earth’s atmosphere. The GNSS-RO uses the several high
gain antennas, with digital beam forming to enable the occultation
measurement of signals with lower level.

The GNSS-RO receiver has a
configurable digital processing section enabling processing of multiple
combinations of GNSS signals. It is able to track not only GPS but also
GLONASS and can be configured to track additional GNSS signals. Most of
the low-level signal processing will be done inside multiple
reconfigurable FPGAs, which can be updated postlaunch to track new
in-band GNSS signals as they become available.

The ability to track multiple GNSS
satellite signals allows the capability to operate during the
transition to GPS-III and past the 2020 retirement of the legacy
signals. This capability significantly improves the quality and
quantity of the radio occultation measurements from previous missions.
The expected instrument data rate is about 53 kbit/s.

REM (Radiation Environment Monitor)

The REM instrument has been
baselined lately in 2016 as a payload experiment for S-6. The REM is
installed externally on the fore panel and provides all elements
necessary to monitor in flight protons, electrons, and heavy ions
fluxes.

Payload performances of Sentinel-6

Primary mission expected
performances: As indicated, the general end-user requirement for S-6 is
to "perform at least as well as Jason-2" in terms of RMS Error (RMS-E)
in the retrievals of SSH (Sea Surface Height), SWH (Significant Wave
Height) and wind speed. This requirement was broken down into the
individual components that make up the measurement of SSH: altimeter
range, orbital altitude, atmospheric corrections, and sea state bias.
An analysis was done on the current state of the art, expected
performances of the POS-4 altimeter, and current POD performances led
to the establishment of the S-6 requirements listed in Table 3, with
some more challenging goals to be met for all products later in the
mission. Overall, the S-6 requirements for the RSS (Root Sum Square)
sea surface height error for LRM measurements closely meet the
established Jason-2 performances, whereas SAR measurements will clearly
outperform Jason-2, because of the reduction in measurement noise. The
only exceptions are the orbit performances, which are kept
conservatively similar to the Jason-3 requirements. However, the
performance goals of orbit determination are likely to be met and are
at least equal to Jason-3 performances.

Although
the requirements for SWH, wind speed, and backscatter have been kept
somewhat less restrictive than the claimed Jason-2 performance, they
are still vastly tighter than the requirements for Jason-3 and
Sentinel-3, which are regarded as far too cautious.

Table 8: Overview of the requirements and actual performances of Jason-2 (NASA, 2011), the requirements for Jason-3 (Couderc, 2015) 35), the requirements for the Sentinel-3 SRAL (Ferreira, 2009; Donlon, 2011) 36)37)
and the requirements and goals for S-6/Jason-CS. In each column either
a single value is presented if it applies equally to NRT, STC, and NTC.
If a triplet of numbers is given, it applies to NRT/STC/NTC. Numbers
are in centimeters, unless indicated otherwise.

Legend to Table 8:
a After ground processing, averaged over 1 s, for 2 m wave height. b
Goals from CNES system performances budget study. c Derived from Ku-
and C-band range difference, averaged over 200 km. d Equal to Jason-2
actual performance. e Could also be expressed as 1% of SWH. f The RSS
values for the NTC products given in (Ref.35) have been corrected in (Ref. 9)
. g NRT/OGDR orbit from real-time DORIS on-board ephemeris. h Whichever
is greater. i After calibration to Jason-1. j After cross-calibration
with other altimeter missions. k For 0.5–8 m SWH range. l For
3-20 ms-1 wind speed range.

RO (Secondary Mission) expected performances

The GNSS-RO will observe
occultations over the SLAT (Straight line Tangent Altitude) range from
- 300 to 500 km, where the SLAT is the minimum elevation above the
reference ellipsoid of an imaginary straight line connecting S-6 and
the occulting GNSS satellite. This is negative in the lower atmosphere
since the refraction bends the ray behind the horizon. As a secondary
payload, the GNSS-RO will not be able to observe the upper atmosphere
up to orbit altitude due to data size limitations.

The occultation tracking rates are
50 Hz for GPS and 100 Hz for GLONASS in the lower atmosphere, while
higher up a 1 Hz tracking is foreseen. Open loop tracking is enabled
from a configurable SLAT altitude downwards. With no ultrastable
oscillator available, occultation processing will rely on single
differencing with respect to a reference GNSS satellite to be tracked
simultaneously.

Based on simulations with a
constellation of 31 GPS and 24 GLONASS satellites and assuming an
antenna coverage of ± 55º in azimuth, the S-6 satellite
will be observing about 1100 occultations per day, about 600 from GPS
and about 500 from GLONASS. Contrary to e.g. the EPS (EUMETSAT Polar
System) and the EPS-SG (EPS-Second Generation), S-6 will fly in a
non-sun-synchronous orbit, providing measurements at various local
solar times, cycling through a full 24 h every 118 days.

Product Processing and Evaluation

Interleaved SAR mode: As
indicated, the Poseidon-4 radar altimeter system can operate in
conventional pulse-width limited (LRM) and SAR processing
simultaneously. Hence, both Brown echoes and SAR radar echoes will be
generated simultaneously in the ground processing. This is loosely
called the interleaved operating mode, because the transmit and receive
pulses are “interleaved” just like in LRM altimetry but at
a much higher rate (9 kHz), Figure 23.
This is in contrast to the burst mode operation of CryoSat-2 and
Sentinel-3, which transmit and receive alternatively, each
approximately one-third of the time. This high rate interleaved pulsing
of the Jason-CS altimeter has the following advantages:

• The original (Jason-2 and -3)
low-resolution processing is maintained simultaneously to
higher-resolution products, thereby ensuring full continuity of
services with Jason-3, based on pulse-width limited processing with an
along-track resolution of approximately 7 km.

• The range noise of SAR
processed altimeter echoes will be reduced by a factor of 1.7 compared
to Sentinel-3 since more independent echoes are received owing to the
continuous pulsing of Jason-CS compared to the burst mode of Sentinel-3
(and CryoSat-2).

• The availability of much
higher along-track resolution (approximately 300 m) and, when averaged,
a lower range measurement noise will enable an enhanced use especially
in coastal areas.

• This enables continuous and
direct comparison of LRM and SAR measurements (which is neither
available from Sentinel-3 or CryoSat-2) and makes Sentinel-6 a
reference for all SAR altimetry missions.

Thanks to the interleaved operating
mode, S-6 will bring some unique opportunities for cross-calibrating
and cross-validating LR and HR altimetry, housed on the same platform,
working from the same altimeter echoes, just using different processing
techniques. Also, it will be the first time that we will be able to
fully process on-ground 100% of the echoes that would otherwise be
averaged on-board.

Once received on ground, the raw
data from the mission will be processed by the responsible institutions
into NRT (Near Real Time) data and other products (Geo Physical Data
Records) which are then distributed to the operational users. In
addition to the European Space Agency, ESA, the French Space Agency,
CNES, was involved in the previous missions as well as the
world’s weather and climate forecasting agencies EUMETSAT and, in
view of the transatlantic cooperation, NOAA. They are also responsible
for the establishment of forecasts of long term changes which affect
climate and society (e.g. agriculture).

On the contrary of the NRT product,
off-line products need longer post processing. Off-line means few days
or weeks after data take. For Sentinel-6, NRT data will be downlinked
at every X-band contact. NRT data will not be older than 3 hours under
the provision that the ground station network setup (which is under
customer responsibility) will be compliant.

Altimeter and Radio Occultation Product Levels: Different levels of products are distinguished in terms of readiness in the various stages of the processing:

• Level 0 products consist of
raw data after restoration of the chronological sequence for each
instrument and removal of data overlaps at dump boundaries and relevant
quality flags related to the reception and decoding;

• Level 1 products maintain the
same time structure and sampling as the Level 0. The instrument
measurements are converted into recognized engineering units. For what
regards the Altimeter, calibration data (radiometric and spectral
calibration as well as geometric registration to geodetic Earth
coordinates) are appended (Level 1a) or applied (Level 1b). Geolocation
data are also appended.

In case of S-6, Level 1a products
will include all the recorded individual echoes in the time domain,
whereas Level 1b provides the synthesized waveforms (without
geophysical corrections). A Level 1b-S product, similar to what is
produced for Sentinel-3 is not envisioned; however, software will be
provided to derive these from the Level 1a product after performing
Delay Doppler processing.

For what regards the Radio
Occultation, at Level 1a, phase and amplitude data as well as the
satellite orbits of the occultation are provided. The Level 1b products
will include the main variables for assimilation, such as the vertical
bending angle profile.

• Level 2 products contains the
geophysical measures, combined with auxiliary input data from other
sources (such as geophysical corrections coming from meteorological
models) to yield directly useful geophysical parameters, e.g. SSH , SWH
and wind speed. The auxiliary data parameters and geophysical
corrections are appended. For altimetry, Level 2 products contain
measurements of SWH, wind speed, and SSH, at a high rate of 20 Hz,
which are then averaged along-track to form one averaged measurement at
1 Hz. These products are thus equivalent to the GDRs (Ground Data
Records) for the Jason missions. — For Radio Occultation at Level
2, temperature and water vapor profiles are provided.

•
Level 2P products are enhanced Level 2 Altimeter products, aimed at
harmonization between missions, e.g. applying the same geophysical
corrections across the missions, or applying externally derived biases
to the data in case they have not been applied yet in the operational
Level 2 products.

• Level 3 products contain
geophysical parameters that have been spatially and/or temporally
resampled or corrected. This may include averaging over multiple
orbits.

• Level 4 products are thematic
data, and are generally gridded parameters that have been derived from
the analysis of the satellite measurements but are not directly derived
from them. These products are elaborated by service providers and users
and are not delivered by the S-6 program.

Product Services and Generation Delays: Based on the synthesis of the operational applications, various product services are identified. Table 9 and Table 10 match the applications with the appropriate product levels. Three different latencies are considered: NRT (Near-Real-Time), STC (Short-Time-Critical), and NTC
(Non-Time-Critical). The latencies govern the quality of the auxiliary
data used in the product generation; therefore better-quality data are
available after a longer elapsed time.

According to the generation latencies, the product services are:

• The Near Real Time Altimetry
service [ALT-NRT, Note: equivalent to the OGDR (Operation Geophysical
Data Record)service in the Jasons] delivers Level 2 products within 3
hours after data acquisition. Because of the reduced time allowed for
the generation, it will often be necessary to rely on alternative data
sources (e.g. predicted or climatological values) for auxiliary data
like altimeter range corrections. The quality of the orbit
determination will also be reduced. Nonetheless, the algorithms used
for the production of Level 2 data from Level 1 are expected to be the
same. In addition, to provide NRT data in the fastest possible way,
data will not be provided in consolidated products with a length of
half an orbit (as is the case for STC and NTC), but will rather be
provided in smaller granules. The main objective of this product
service is to provide information on the sea-state (SWH and wind speed,
but also on SSH). It is mainly used for marine meteorology,
ocean-atmosphere air-sea transfer studies and real-time operational
oceanography.

• Short Time Critical Altimetry
service [ALT-STC, Note: equivalent to IGDR (Interim Geophysical Data
Record) service in the Jasons] delivers Level 2 products within 36
hours after data acquisition which enables consolidation of some
auxiliary or ancillary data (e.g. preliminary orbit determination).
These products will be produced using the same algorithms as the NTC
products and they will have the same data structure. The main objective
is to support operational oceanography, i.e. improve ocean state
analysis, forecasts, and hindcasts produced by NOP (Numerical Ocean
Prediction) systems assimilating sea surface height measurements
derived from a multi-mission constellation of spaceborne altimeters.
Level 3 products contain also geophysical parameters that have been
spatially and/or temporally resampled or corrected. This may include
averaging over multiple measurements. They are primarily intended for
ocean modelling services. At this point, Level 3 data will only be
provided with short time critical latency. This product service is
mainly used for operational oceanography and geophysical studies.

• Non Time Critical Altimetry
service [ALT-NTC, Note: equivalent ot GDR (Geophysical Data Record)
service in the Jasons] delivers Level 2 products within typically 2
months after data acquisition, allowing the further consolidation of
some auxiliary data (e.g. precise orbit data, radiometer calibration)
leading to higher accuracy of SSH products. These products will be
subject to regular reprocessing as better information about
instrumentation biases, precise orbits, and geophysical corrections
become available. The main objective of this product service is to
provide information on ocean topography and mean sea level in support
of ocean and climate monitoring services and it is mainly used for
geophysical studies and operational oceanography.

• Near-Real-Time Radio
Occultation product service (RO-NRT) delivers Level 1b and Level 2
products within 3 hours after sensing, for direct assimilation into NWP
models. It will be provided by the US partners of the program. The main
objective of the RO-NRT product service is to provide bending angles or
refractivity profiles, which contain information on atmospheric
temperature, pressure, and humidity. Further Level 2 products are e.g.
tropopause height, planetary boundary layer height, and ionospheric
information.

• Non-Time-Critical Radio
Occultation product service (RO-NTC) delivers Level 1b and Level 2
products within 60 days after sensing. Longer time series of the
instrument are used to obtain improved precise orbit determination and
clock data, as well as using updated auxiliary data (e.g. precise orbit
and clock data for the GNSS satellites). The main objective of the
RO-NTC is to deliver higher precision version of the NRT data, making
this service particularly valuable for climate studies, including
assimilation in reanalysis models. Two parallel services will be
providing these data. They both start from Level 0 and thus allow
estimating uncertainties introduced by the processing set-up. On the
European side, processing up to Level 1b is performed at EUMETSAT and
the ROM SAF (Radio Occultation Meteorology Satellite Application
Facility) is responsible for processing these data further to Level 2
(and also to Level 3 within their climate service). The other RO-NTC
service will be provided by the US partners of the program.

The
naming of these services (characterized by product latencies) are in
common with those of the Sentinel-3 ocean surface topography mission.

Application category

NRT

STC

NTC

Product level

Level 1

Level 2/3

Level 1

Level 2/3

Level 1

Level 2/3

Marine meteorology

-

+

-

-

-

-

Operational oceanography

-

+

-

+

-

+

Climate change

-

-

-

-

-

+

Research and remote-sensing science

-

+

+

Table 9:
Mapping of the main application areas on the altimetry product
services(Level 1 and Level 2). The mapping for Level 3 products is
equivalent to the one of the Level 2 products (+ is essential; is
beneficial; - is less important)

Application Category

NRT

NTC

Product level

Level 1

Level 2

Level 1

Level 2

Numerical weather prediction

+

+

-

-

Climate change

-

-

+

+

Research and remote sensing science

+

+

Table 10:
Mapping of the main application areas on the radio occultation product
services (+ is essential; is beneficial; - is less important)

In summary, the Sentinel-6
mission, consisting of two consecutively flying altimeter satellites,
Sentinel-6 A and Sentinel-6 B, will ensure the continuation of the
decades-long time series of sea level as recorded by TOPEX/Poseidon,
Jason-1, Jason-2, and Jason-3, from 2020 onwards. Since the RA (Radar
Altimeter) will be able to serve simultaneously conventional LRM
altimeter, and a SAR altimeter measurements, it does not only provide
compatibility with the previous missions, that is vital for an accurate
cross-calibration, but it will also improve sampling of the coastal
areas with a much higher resolution, and providing the ability to
measure closer to the coast line. Stability and performances of the
measurements will be also improved, wrt the predecessors, to cope with
day by day more demanding scientific needs.

The Sentinel-6 mission will be the
first of the “reference missions” for which a wide range of
Level 1, Level 2P, and Level 3 products will be provided. These are not
only aiming at operational meteorological and oceanographic modelers,
but are also giving the altimeter specialist the opportunity to advance
further altimeter technologies that will be provided by the unique
interleaved mode altimeter flown on the Sentinel-6 satellites.

Sentinel-6 will also include a
secondary radio occultation payload, which makes use of GPS and GLONASS
satellites occultations to measure physical properties of the
atmosphere such as temperature, pressure and water vapor.

Sentinel-6 mission data, measuring
how much heat is in the ocean, pinpointing where it is, and map its
movement through ocean currents, will help the scientific community to
better understand climate and predict future climate change.

The information compiled and edited in this article was provided byHerbert
J. Kramer from his documentation of: ”Observation of the Earth
and Its Environment: Survey of Missions and Sensors” (Springer
Verlag) as well as many other sources after the publication of the 4th
edition in 2002. - Comments and corrections to this article are always
welcome for further updates (herb.kramer@gmx.net).